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The Old River Control Structure (ORCS) is a floodgate system in a branch of the Mississippi River in central Louisiana. It regulates the flow of water leaving the Mississippi into the Atchafalaya River, thereby preventing the Mississippi river from changing course. Completed in 1963, the complex was built by the U.S. Army Corps of Engineers in a side channel of the Mississippi known as "Old River," between the Mississippi's current channel and the Atchafalaya Basin, a former channel of the Mississippi.〔 Republished in 〕〔. Includes map and pictures.〕〔〔 The Old River Control Structure is actually a complex containing the original low-sill and overbank structures, as well as the auxiliary structure that was constructed after the low-sill structure was damaged during the Mississippi River Flood of 1973. The complex also contains a navigation lock and the Sidney A. Murray, Jr. Hydroelectric Station. == Purpose == Before the 15th century, the Red River and Mississippi River were entirely separate and more or less parallel to one another.〔(【引用サイトリンク】url=http://www.americaswetlandresources.com/background_facts/detailedstory/LouisianaRiverControl.html )〕 Beginning in the 15th century, the Mississippi River created a small westward loop, later called Turnbull's Bend, near present-day Angola, Louisiana. This loop eventually intersected the Red River, making the downstream part of the Red River a distributary of the Mississippi; this distributary came to be called the Atchafalaya River.〔 In the heyday of steamboats along the Mississippi River, it would take a boat several hours to travel the 20 miles of Turnbull's Bend, after which it would have progressed only a mile or so from the entrance to the bend. To reduce travel time, Captain Henry M. Shreve, a river engineer and founder of Shreveport, La., dug a canal in 1831 through the neck of Turnbull's Bend; this canal became known as Shreve's Cut. At the next high water, the Mississippi roared through this channel. With the Mississippi River taking a new course, the Red River began emptying into the smaller Atchafalaya River. As well as this, the Atchafalaya River drained additional water from the Mississippi River through the abandoned Turnbull's Bend, which had come to be known as "Old River”. With this extra intake of water, the channel of the Atchafalaya River was worn deeper and wider throughout the 1800s and early 1900s.〔 The U.S. Army Corps of Engineers measured the amount of water flowing through the Mississippi River and compared it to the amount entering the Atchafalaya Basin by monitoring "latitude flow" at the latitude of the Red River Landing, located five miles downstream of Old River. In this case, latitude flow is a combination of the flows of the Mississippi and Atchafalaya rivers as they cross an imaginary line at that latitude.〔 Between 1850 and 1950, the percentage of latitude flow entering the Atchafalaya River had increased from less than 10 percent to about 30 percent. By 1953, the U.S. Army Corps of Engineers concluded that the Mississippi River could change its course to the Atchafalaya River by 1990 if it were not controlled, since this alternative path to the Gulf of Mexico through the Atchafalaya River is much shorter and steeper.〔 The Corps completed construction on the Old River Control Structure in 1964 to prevent the main channel flow of the Mississippi River from altering its current course to the Gulf of Mexico through the natural geologic process of avulsion.〔 Historically, this natural process has occurred about every 1,000 years, and is overdue. Some researchers believe the likelihood of this event increases each year, despite artificial control efforts. If the Mississippi diverts its main channel to the Atchafalaya Basin and the Atchafalaya River, it would develop a new delta south of Morgan City in southern Louisiana, greatly reducing water flow to its present channel through Baton Rouge and New Orleans.〔(【引用サイトリンク】title=Will the Mississippi River change its course in 2011 to the red line? )〕 The Mississippi Flood of 1973 almost caused the control structure to fail. Integrity of the Old River Control Structure, the nearby Morganza Spillway, and other levees in the area is essential to prevent such a diversion. Jeff Masters of ''Weather Underground'' noted that failure of that complex "would be a serious blow to the U.S. economy." 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Old River Control Structure」の詳細全文を読む スポンサード リンク
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